The present invention relates to a method of and an apparatus for purifying an exhaust gas discharged from an internal combustion engine such as a vehicle engine and, more particularly, to a method of and apparatus for purifying an exhaust gas discharged from an internal combustion engine capable of operating at a lean air-to-fuel ratio (lean burn) or a vehicle mounting the internal combustion engine capable of operating at a lean air-to-fuel ratio.
Carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NO.sub.x) contained in an exhaust gas discharged from an internal combustion engine such as a vehicle engine cause problems in that a harmful influence is affected on human as air pollution materials and plants are inhibited from growing.
Therefore, considerable efforts have been made to reduce an emission amount of the air pollution materials. Fruitful successes have been being attained step-by-step by developing methods of purifying the exhaust gas using a catalyst or the like in addition to reducing a production amount of the air pollution materials by improving combustion methods of an internal combustion engine. In regard to a vehicle mounting a gasoline engine, a main method of rendering the exhaust gas harmless has been that oxidation of HC and CO and deoxidation of NO.sub.x are simultaneously performed using a three way catalyst containing main activation components of Pt, Rh.
An exhaust gas purifying function of the above-mentioned three way catalyst effectively acts only to an exhaust gas generated by burning under a condition near the theoretical air-to-fuel ratio (stoichiometric air-to-fuel ratio) called a window due to its characteristics. Therefore, in the conventional engine, although the air-to-fuel ratio is changed depending on a driving condition of the vehicle, the air-to-fuel ratio has been essentially limited to a changing range near the stoichiometric air-to-fuel ratio (in a case of gasoline, A (amount of air in weight)/F (amount of fuel in weight)=approximately 14.7; hereinafter, the stoichiometric air-to-fuel ratio is represented by A/F=14.7. However, the value varies depending on a kind of the fuel.).
In recent years, a so-called lean burn combustion technology has been developed and it has been not rare to see vehicles having an engine operated under a lean burn condition above an air-to-fuel ratio of 18 because fuel consumption rate can be improved by operating an engine at an air-to-fuel ratio leaner than the stoichiometric air-to-fuel ratio. However, if purifying of the exhaust gas from the lean burn combustion is performed using the three way catalyst used at present as described above, NO.sub.x cannot be effectively purified by reduction though HC and CO can be purified by oxidation.
Accordingly, in order to expand lean burn combustion time (to expand an operating range of applying the lean burn combustion method) by mounting a lean burn combustion engine on a large vehicle, an exhaust gas purifying technology capable of coping with the lean burn combustion is required. Therefore, development of the exhaust gas purifying technologies capable of coping with the lean burn combustion, that is, the technologies of purifying HC, CO, NO.sub.x in an exhaust gas containing a large amount of oxygen (O.sub.2), particularly development of the technologies of purifying NO.sub.x are extensively being progressed.
One of the technologies which have been proposed is that NO.sub.x is separated from a lean burn exhaust gas using an NO.sub.x absorbent (at least, separated from O.sub.2 in the exhaust gas), and then the NO.sub.x separated by the NO.sub.x absorbent is reduced to N.sub.2 to be rendered harmless and the NO.sub.x absorptive capability of the NO.sub.x absorbent is recovered by catalytic reaction of the NO.sub.x with a reducing agent such as a hydrocarbon, carbon monoxide, hydrogen or the like.
For example, in technologies disclosed in Japanese Patent Application-Laid Open No. 62-97630, Japanese Patent Application-Laid Open No. 62-106826 and Japanese Patent Application-Laid Open No. 62-117620, NO.sub.x in an exhaust gas (after NO is converted into NO.sub.2 which is easily absorbed) is removed to be absorbed by being brought in contact with a catalyst having an NO.sub.x absorptive capability. When the absorbing efficiency is decreased, the exhaust gas is stopped to pass through the catalyst and the accumulated NO.sub.x is removed by being reduced using a reducing agent such as H.sub.2, HC produced from methane, gasoline or the like to recover the NO.sub.x absorptive capability of the catalyst.
On the other hand, in a technology disclosed in Japanese Patent No. 2600492, in an exhaust gas passage of an engine there is provided an NO.sub.x absorbent which can absorb NO.sub.x, when the exhaust gas is discharged during lean operation and can discharge the absorbed NO.sub.x when the oxygen concentration in the exhaust gas is decreased. Thereby, NO.sub.x is absorbed when the exhaust gas is discharged during lean operation and the absorbed NO.sub.x is discharged by decreasing the O.sub.2 concentration in the exhaust gas flowing into the NO.sub.x, absorbent to reduce the NO.sub.x to N.sub.2.
The technologies described above have the following problems to be solved.
That is, the first problem is what reducing agent is used for reducing and rendering harmless the NO.sub.x absorbed and captured in the catalyst. Although the merit of a vehicle mounting a lean burn combustion engine is in energy saving based on improvement of fuel consumption rate as described above, using of the reducing agent decreases the improvement of fuel consumption rate. Therefore, an amount of the reducing agent should be minimized as small as possible, and the apparatus and the control for using the reducing agent should be simple and reliable. In addition to these, employing of the system should not deteriorate performance and operability of the vehicle.
Secondary, in the technologies disclosed in Japanese Patent Application-Laid Open No. 62-97630, Japanese Patent Application-Laid Open No. 62-106826 and Japanese Patent Application-Laid Open No. 62-117620 described above, an amount of the reducing agent burnt by O.sub.2 in the exhaust gas is suppressed to reduce an amount of the reducing agent in use because the exhaust gas is stopped to pass through the catalyst and the reducing agent such as HC is brought in contact with the NO.sub.x absorbent in order to recover the NO.sub.x absorbent. However, two NO.sub.x absorbent units need to be provided and an exhaust gas switching mechanism for alternately conducting the exhaust gas through the units is also required. Therefore, there is caused a problem in that the structure of the exhaust gas purifying apparatus becomes complex.
Thirdly, in the technology disclosed in Japanese Patent No. 2600492, the exhaust gas is always conducted through the NO.sub.x absorbent, and NO.sub.x is absorbed when the exhaust gas is discharged under a lean burn condition and the absorbed NO.sub.x is discharged to recover the absorbent by decreasing the O.sub.2 concentration in the exhaust gas (combustion under a rich burn condition). Therefore, switching of the gas flow is not necessary and accordingly the above second problem can be solved.
However, it is based on the premise that the material is employed as the catalyst, which material can absorb NO.sub.x when the exhaust gas is discharged during lean operation and can discharge the absorbed NO.sub.x when the oxygen concentration in the exhaust gas is decreased. The above-mentioned technology discloses a means for discharging the exhaust gas under combustion in a rich burn condition when the discharged NO.sub.x is reduced. However, in order to actually mount the above-mentioned means on the vehicle, it is necessary to optimize the condition in the rich combustion, taking into consideration the fuel economy (fuel consumption rate), the operability, the driveability, the reliability and so on. Therefore, from the standpoint of optimization of the fuel economy, the problem is how the amount of fuel consumed in the rich combustion is decreased. In addition to this, the optimization closely relates to the characteristic of the NO.sub.x absorbent, and accordingly selection of the NO.sub.x absorbent is also one of the important elements.